MTL Annual Research Report 2011

J-aggregates, which are self-assembling molecular nanostructures ^[1] , have unique optical properties resulting from coherent coupling of their molecular components. For instance, J-aggregate films a few nanometers thick can strongly absorb light, making them interesting candidates for applications involving strong coupling in a microcavity ^[2] . In a J-aggregate, the peak absorption wavelength is red-shifted and narrowed with respect to the peak absorption wavelength of its molecular components due to dipole-dipole coupling, which produces a collective excitation, called an exciton, that is delocalized over the nanostructure ^[3] . Static and dynamic disorder in the aggregate affects the number of molecules over which the exciton is delocalized, therefore broadening the absorption line width. We use 2D Fourier transform optical spectroscopy to quantify the exciton size and determine its relationship to inhomogeneous (static) and homogeneous (dynamic) broadening.

Figure 1(a) shows a 2D correlation spectrum of a film of BIC ^[4] J-aggregates deposited on a sapphire substrate and cooled to 6 K in a cryostat. The negative-going (blue) feature indicates bleaching of the exciton transition while the positive-going (red) feature indicates excited state absorption to a two-exciton state. The energy separation of the two features is related to the exciton size ^[5] , similar to the model for a quantum-mechanical “particle-in-a-box.” Figure 1(b) shows the dependence of the exciton size on its absorption energy. Figure 1(c) shows that the average exciton size given by the pump-probe spectrum (17 molecules) does not reflect the value of the energy-dependent delocalization since the pump-probe peaks are broadened due to the inhomogeneity of the film.  As the temperature increases, the exciton size decreases, as Figure 2(a) shows, due to increased exciton-phonon scattering. The homogeneous line width also increases as the temperature increases, as in Figure 2(b), while the inhomogeneous line width remains constant, indicating that dynamic disorder influences the exciton size more strongly.